One of our research emphases is to understand GPCR (G protein-coupled receptor) signaling mechanisms and functions. In a siRNA screen, we unexpectedly discovered that Serine/Threonine Kinase (STK) 24 and its binding partner Cerebral Cavernous Malformation (CCM) 3 in regulation of neutrophil degranulation. STK24, together with MST4 and STK25, belongs to the germinal center kinase (GCK) III sub-family of Sterile-20 kinases. These kinases are known to interact with CCM3. CCM3 has been directly, whereas the GCK III kinases indirectly, implicated in the CCM disease. CCM is a vascular pathological condition that affects the vasculature of the central nervous system and results in stroke, seizure and cerebral hemorrhage. Despite the potential importance of the STK-CCM3 complex, their cellular functions and underlying biochemical mechanisms remain obscure. Following our discovery of their role in degranulation, we identified CCM3 and STK24 interaction with an exocytosis regulatory protein, UNC13D, through the mass spectrometry (MS) analysis. Furthermore, we have gathered solid preliminary results to help us understand how STK24 and CCM3 mechanistically regulate the degranulation and how extracellular stimuli, including GPCR ligands, regulate the release of STK24/CCM3-controlled granule pools via Ca2+. Thus, our findings provide the first insights into a cellular function of the STK24-CCM3 complex with well- defined biochemical basis. Moreover, our present study has unraveled previously unknown roles of STK24 and CCM3 in the regulation of neutrophil degranulation, a process important for acute innate immune responses and tissue damage that occurs in chronic and systemic inflammation and ischemia-reperfusion, but understudied. Furthermore, our new mechanism for regulation of degranulation would facilitate the general understanding of ligand-stimulated exocytosis, which degranulation is a form of and occurs in many cells including endothelial cells. Thus our study may pave the way for the eventual understanding of the pathogenic basis for the CCM disease and exert potentially high impact on a number of fields (basic cell biology, innate immunity, and CCM disease). In the present study, we will focus on neutrophil degranulation by testing the hypothesis that the STK24 and CCM3 complex is a key negative regulator of neutrophil degranulation. We will investigate the biochemical and molecular mechanisms by which CCM3 and STK24 regulate degranulation.